Figures 331 (left) and 332 (right) from Chapter 14 (Sperm Flagellum) of 'The Cell, 2nd Ed.' by Don W. Fawcett M.D. Left, longitudinal section of midpiece from sperm of the suni antelope; right, longitudinal section through the principal piece of mouse sperm. A PDF copy of the accompanying chapter is available on the ASCB’s BioEDUCATE website.